Apparatus and method for resisting tear propagation in polymeric products

ABSTRACT

A plastic bag for holding materials comprising at least one polymeric panel defining the outer walls of the plastic bag. The at least one polymeric panel is interconnected to form a substantially cylindrical shape. The at least one polymeric panel defines a machine direction and a cross direction perpendicular to the machine direction. A plurality of substantially parallel rows on the at least one polymeric panel wherein the plurality of substantially parallel rows is comprised of a plurality of curvilinear areas. The plurality of curvilinear areas having a protrusion or an indentation in the at least one polymeric panel. The bag further comprises a plurality of substantially parallel neutral areas wherein each substantially parallel neutral area is defined by an area between two adjacent rows in the plurality of substantially parallel rows.

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX

Not applicable.

FIELD OF THE INVENTION

The present invention relates to products constructed from plastic sheets, in particular plastic bags, the products having indentions and protrusions configured in such a way as to provide enhanced resistance to tear propagation without effectively altering the elasticity of the plastic sheets.

DESCRIPTION OF THE RELATED ART

Plastic bags are utilized throughout the world for refuse collection, storage, and numerous other purposes. Plastic bags are generally manufactured using a blown-film extrusion process which comprises forming a blown-film tube from polyethylene or other polymeric materials, flattening the blown-film tube, and forming a set of seals and/or cuts to the blown-film tube to form a plurality of plastic bags from the blown-film tube.

In manufacturing bags using the blown-film extrusion process, a tubular film is formed with air that blows through the center of the tube as heated molten plastic is extruded through a die on the extrusion machine. The resulting tubular film is of substantially uniform thickness around the entire circumference of the film. A pair of nip rollers pulls the blown-film up and the film cools as it continues to be pulled upward. As the film cools, it can be eventually be flattened and processed into various types of bags or other plastic products.

One method of manufacturing plastic bags is by first providing sets of closely spaced, parallel transverse seals at predetermined intervals along the length of a flattened blown-film polymeric tube. A transversely extending line of perforations is provided between the closely spaced, parallel seals. These perforations form the sides of the plastic bags. The bottom of the plastic bag is usually formed from the fold of the blown-film tube as it is flattened. Subsequently, another cut is provided along the central, longitudinal axis of the flattened blown-film polymeric tube to form the top openings of these bags.

Plastic bags may also be produced by providing a pair of closely spaced, parallel seals extending transversely at predetermined intervals along the length of a flattened blown-film polymeric tube. A line of perforations extending transversely across the flattened blown-film polymeric tube is introduced between the pair of seals. In this case, each seal forms the bottom of the bag, and another perforation line is introduced at a point somewhere between successive pairs of closely spaced, parallel seals to form the opening, or top, of the bags. These bags typically result from the use of the entire circumference of the blown-film polymeric tube.

The blown film extrusion process results in bags that generally have a uniform thickness. There is an inherent relationship between the strength and thickness of the plastic bag. As the thickness of the bag is reduced, it becomes lighter and generally less expensive to manufacture.

One of the most significant limitations in offering thinner bags is that thinner bags are generally more prone to ripping and tearing. It is known that plastic bags formed from the blown-film extrusion process tend to offer differing levels of resistance to tear propagation depending on the direction of the tear. In particular, the blown-film polymeric tube is more susceptible to tearing in the machine direction, which is the longitudinal axis of the blown-film polymeric tube, than in the cross direction, which extends across the width of the blown-film polymeric tube, perpendicular to the machine direction. This is primarily due to the molecular orientation of the film resulting from the blown-film process.

Plastic sheet products are susceptible to stretching because of the natural elastic properties of plastics in general, especially when considering the thin plastic used to manufacture bags. When an object within a bag forces the walls of the bag to stretch to a point where they can no longer resist the force of the object, a tear or puncture is formed. One method to attempt to address this problem used in the prior art is to modify the bags so that they controllably stretch in a manner that exceeds their natural elastic properties. In such cases, when a protruding object inside or outside of a plastic bag comes into contact with the walls, it causes the walls of the bag to stretch “controllably.” As a result, the amount of plastic material in the wall of the bag stays constant while the surface area of the bag increases, causing the thickness of the walls to be significantly reduced. Therefore, even though a tear has not formed in the bag, the structural integrity of the bag has been significantly diminished. As an alternative to “controlled stretching,” it would be desirable to offer a plastic bag that offers properties of increased resistance to tear propagation without increased stretching of the plastic bags and without diminishing the structural integrity of the bag.

One of the problems with tears in plastic bags, or when any other type of puncture is introduced to a plastic sheet, is that even a small tear or puncture in the bag has a tendency to propagate along the machine direction of the bag, which, as discussed previously, is typically markedly weaker than the cross-direction. As noted above, bags manufactured with a perforation forming the side of the bag have the weaker machine direction running across the width of the bag, while the stronger cross direction runs from the bottom of the bag to the top of the bag. In such bags, it would be desirable to redirect a tear propagating in the weaker machine direction to turn in the cross-direction, which is stronger and therefore more resistant to tear propagation.

With bags that are manufactured using the entire circumference of the blown-film polymeric tube, the width of the bag tends to be the stronger cross direction and the machine direction tends to run in the vertical direction from the top of the bag to the bottom of the bag. In these bags, tears have a tendency to propagate in the vertical direction, which is the weaker machine direction. In these bags, it is desirable to redirect this tear into the horizontal, cross direction of the bag, which is stronger and more resistant to tear propagation. Doing so would effectively result in increased tear resistance of the bag as a whole.

In previous methods, attempts to control tear propagation were made by introducing, through an embossing process, a plurality of connected diagonal lines into the wall of the bag, i.e. a linear direction. The lines have the effect of redirecting a tear propagating in the machine direction along one of the diagonal lines. Redirecting the tear in a direction that is roughly oriented at a 45 degree angle to both machine direction and cross direction of the bag can result in an increased resistance to tear propagation. However, in the event a tear is naturally propagating in the cross-direction, the lines can actually and undesirably encourage the tear to propagate into the weaker diagonal machine direction.

By using diagonal lines to attempt to direct the propagation of a tear, the diagonal lines are interconnected so that after a short distance a line runs into another perpendicular diagonal line. Ideally, the tear would immediately turn and follow the perpendicular diagonal line. However, in practice the hard angle between the two lines rarely results in such, and generally results in a tear that breaks free of the diagonal path because the force is concentrated into the apex formed by the junction angle of the two lines, allowing tearing in the machine direction until it encounters another diagonal. Therefore, it would be desirable to introduce more gradual transitions and turns in plastic sheets, such as a plastic bag, to encourage a propagating tear to follow a curved and tortuous path instead of one comprised of diagonal lines.

SUMMARY OF THE INVENTION

The present disclosure introduces a novel way of addressing the problems and issues involved in providing increased resistance to tear propagation while not effectively altering the stretching or elastic properties of the plastic bag. In particular, the present disclosure does not induce properties in the bag or plastic sheet that would cause the bag to be more likely to stretch when a force is applied against one of its walls.

The present disclosure generally relates to the introduction of a plurality of indentions and protrusions into a plastic bag, which as previously mentioned is typically formed using a blown-film extrusion process. The plurality of indentions and protrusions are curvilinear in shape, such as elliptical or S-shaped. The curvilinear shapes lack sharp corners, thereby providing increased resistance to force concentration and resulting in increased tear propagation resistance as more fully described in the drawings and figures. The curvilinear shapes are generally configured so as to be adjacent to one another with a series of curvilinear shapes forming a row of curvilinear shapes in the cross direction of the plastic bag or plastic sheet.

While the present disclosure is directed primarily toward plastic bags, it is contemplated that novel techniques and structures disclosed herein have uses outside the realm of plastic bags. In particular, other types of plastic sheeting products manufactured using a blown-film extrusion process may benefit from the novel features discussed herein. This is especially true since the plastic bags of the present invention are essentially constructed from a plastic sheet, albeit one tubular in nature.

Another aspect of the present disclosure is a method of manufacturing plastic film having increased resistance to tear propagation without effectively altering the stretching properties of the plastic film. In particular, the method entails using a blown-film polymeric tube having a machine direction along the longitudinal axis of the polymeric tube that is less resistant to tear propagation than the cross direction which is perpendicular to the machine direction. After flattening the blown-film polymeric tube, the tube is embossed with a plurality of curvilinear indentations and protrusions, the plurality of curvilinear indentations and protrusions defining a plurality of substantially parallel rows extending in the cross direction.

The present disclosure is described primarily with reference to plastic bags; however, it is recognized that numerous other types of plastic sheeting products would benefit from the methods described herein. Therefore, the present disclosure is not limited to the embodiments described herein. The embodiments described herein illustrate the preferred methods and ways of practicing the invention, but these techniques can be applied to other types of plastic sheeting having properties similar to bags. The claims herein are intended to more fully encompass the scope of invention as contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and complete understanding of the present disclosure may be obtained by reference to the detailed description of the invention when viewed with reference the accompanying drawings. The drawings can be briefly described as follows:

FIG. 1A provides a plan view of a side seal plastic bag as known in the prior art, highlighting the machine direction and cross direction of that plastic bag.

FIG. 1B provides a plan view of a bottom seal plastic bag as known in the prior art, highlighting the machine direction and cross direction of that plastic bag.

FIG. 2 illustrates a particular pattern used on panels of plastic bags as known in the prior art.

FIG. 3 illustrates a novel tear-resistant pattern according to one embodiment of the present invention.

FIG. 4A depicts a visual diagram of one method of manufacturing plastic film according to an embodiment of the present invention utilizing a stamping press and complementary anvil.

FIG. 4B is a visual diagram of an alternative method of manufacturing plastic film according to another embodiment of the present invention by utilizing a set of complementary embossing rollers.

FIG. 5A is an illustration of one alternative configuration for arranging the curvilinear protrusions and indentations according to the present invention.

FIG. 5B is an illustration of another alternative configuration for arranging the curvilinear protrusions and indentations according to the present invention.

FIG. 5C is an illustration of yet another alternative configuration for arranging the curvilinear protrusions and indentations according to the present invention.

FIG. 5D is an illustration of a non-uniform alternative configuration for arranging the curvilinear protrusions and indentations according to the present invention.

FIG. 5E is an illustration of another alternative configuration for arranging irregularly shaped curvilinear protrusions and indentations according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure illustrates embodiments of the present invention. In view of the disclosure of the invention contained herein, a person having ordinary skill in the art will recognize that innumerable modifications and insubstantial changes may be incorporated or otherwise included within the present invention without diverging from the spirit of the invention. Therefore, it is understood that the present invention is not limited to those embodiments disclosed herein. Furthermore, the appended claims are intended to encompass the invention to the fullest extent possible, but it is fully appreciated that limitations on the use of particular terms is not intended to conclusively limit the scope of protection for a particular application.

Referring now to FIG. 1A, a perspective view of a side seal plastic bag 100 is provided. The bag has an open top 110 and has a bottom 112 formed by the folded longitudinal edge of the blown-film tube and sealed sides 116. The front and back of the bag 100 is comprised of polymeric panels 114. Bags manufactured in this way have a machine direction 40 that runs horizontally across the bag as depicted and a cross direction 50 that runs vertically across the length of the bag. The cross direction 50 of the polymeric panels 114 is markedly stronger than the machine direction 40; therefore, the plastic bag has a tendency to resist tearing in the vertical cross direction 50 to a much greater extent than in the horizontal machine direction 40.

Referring now to FIG. 1B, a perspective view of a bottom seal plastic bag 150 is provided. The bag 150 has an open top 110 and a single sealed bottom 112. The entire tubular material of a blown-film polymeric tube is used to form this plastic bag as previously described, so there is a single continuous polymeric panel 114 that forms the outer wall of the bag 150. In these types of constructions, the bags 150 have a machine direction 40 that runs in the vertical direction from the top 110 of the bag 150 to the sealed bottom 112. The cross direction 50 of the bag 150 runs horizontally around the circumference of the bag 150. As with the side seal bags 100, the cross direction 50 of the polymeric panel 114 is markedly stronger, and more tear-resistant, than the machine direction 40. Accordingly, this bag 150 has a tendency to resist tearing in the horizontal cross direction 50 much more so than in the vertical machine direction 40.

Referring now to FIG. 2, an example of a prior art attempt to resist tear propagation is illustrated. The prior art straight-line system 200 consists of a plurality of first diagonal lines 210 and a plurality of second diagonal lines 214 running in a perpendicular fashion to the plurality of first diagonal lines 210. In this particular illustration the machine direction 40 runs from left to right while the cross direction 50 runs top to bottom. However, because of the symmetry of the prior art straight-line system 200, if the cross direction 50 and machine direction 40 were reversed, the prior art straight-line system 200 would function in basically the same manner. This can best be seen by rotating FIG. 2 in the clockwise or counter-clockwise directions. Each of the first diagonal lines 210 has a finite length and runs between a pair of perpendicular second diagonal lines 214. In this particular prior art straight-line system 200 the plurality of first diagonal lines 210 and the plurality of second diagonal lines 214 terminate when they intersect, never forming a four-way junction. Each intersection of a first diagonal line 210 with a perpendicular second diagonal line 214 forms a T-shaped intersection 212.

In the prior art straight-line system 200, a tear can be contemplated propagating in the machine direction 40, which in this particular illustration would be a tear running from left to right. Ideally, the tear would follow one of the first diagonal lines 210 or second diagonal lines 214. Assuming it propagates along one of the first diagonal lines 210, when the tear reaches a T-shaped intersection 212 with a perpendicular second diagonal line 214, it is hoped that the tear will continue propagating down the second diagonal line 214. However, even in this case, that is not what occurs.

FIG. 3 illustrates one embodiment of the present invention. In particular, it shows a magnified view of one configuration for a portion of the plastic sheet 114 used in the manufacture of the bags 150, 100 illustrated in FIG. 1A and FIG. 1B. In this embodiment, a row embodiment of ellipses 300, the plastic sheet 114 of the bag is configured with a plurality of curvilinear areas, particularly, raised elliptical areas 310 and recessed elliptical areas 312. The process used to introduce the raised elliptical areas 310 and recessed elliptical areas 312 can be an embossing process, a debossing process, a combination of the two, or any other method known in the art. These methods will be better illustrated with reference to FIG. 4A and FIG. 4B. Referring back to FIG. 3, the raised elliptical areas 310 and recessed elliptical areas 312 form a plurality of rows 330, 332, 334. Each row 330, 332, 334 extends in the cross direction 50 of the plastic sheet 114. Between two adjacent rows 330, 332, 334 is a neutral area 320, 322 that also extends in the cross direction 50 of the plastic sheet 114

With specific reference to elliptical row embodiment 300 of FIG. 3, it is readily determined that a tear, depicted as 70, propagating in the machine direction 40, i.e. from the top to the bottom of the figure, will encounter the raised elliptical areas 310 or recessed elliptical areas 312. Testing has shown that a tear 70 has a tendency to attempt to propagate along the perimeters of the raised elliptical areas 310 or recessed elliptical areas 312. As the tear 70 attempts to propagate across a first row 330 it will follow along the curved edge of the raised elliptical area 310 or recessed elliptical area 312. Eventually, the tear 70 will have a tendency to diverge from the raised elliptical area 310 or recessed elliptical area 312.

However, as the tear has a tendency to attempt to propagate around the edges of the elliptical areas 310, 312, the tear is now redirected generally in the cross direction 50 of the plastic sheet 114 rather than the machine direction 40. At this point, the tear has a natural tendency to try to revert back to the path of least resistance, i.e., the machine direction 40. However, as the tear attempts to revert back to the machine direction 40, it encounters another row 332 of raised elliptical areas 310 and recessed elliptical areas 312. This process continues and frustrates the attempts of the tear to propagate. Tests show that this configuration can increase the resistance to tear propagation by 15% or more.

Regarding the scale of FIG. 3, the size and depth of the raised elliptical areas 310 and recessed elliptical areas 312 are extremely small. In fact, in some embodiments, the elliptical areas 310, 312 have major axis lengths, or row 330, 332, 334 widths, of 1 mm or less. The net result is that it is extremely difficult to distinguish the individual rows 330, 332, and 334, and neutral areas 320 and 322 during normal observation. In fact, a person must make a very close inspection to even recognize the raised elliptical areas 310 or recessed elliptical areas 312 and the scale makes the neutral areas 320, 322 almost indiscernible with the naked eye in normal use. The resulting plastic sheet 114 has a unique textured appearance.

Referring now to FIG. 4, one method of embossing by stamping 400 is illustrated. In this particular illustration, a roll of polymeric material 410 to be embossed is oriented by a roller 402 to position the polymeric material 410 between a stamping press 420 and an anvil 422. With a combination of heat and pressure, the stamping press 420 is extended downward (as illustrated by arrow 421) to force the polymeric material 410 into the anvil 422 under the stamping press 420. The stamping press 420 and anvil 422 have complementary configurations that result in the patterns such as those in the present disclosure. Embossing/debossing of patterns with a stamping press 420 is generally well-understood as is its implementation. However, one disadvantage is that the polymeric material 410 must be halted to allow the stamping press 420 and anvil 422 to operate.

An alternative to the method of FIG. 4A is the rolling embossing system 450 illustrated in FIG. 4B. In the rolling embosser system 450, the polymeric material 410 is oriented by a roller 402 to travel between an upper embossing roller 460 and a lower embossing roller 462. The upper embossing roller 460 and the lower embossing roller 462 have complementary configurations, i.e. an indentation on the upper embossing roller 460 matches a protrusion on the lower embossing roller 462 which will form a raised area, such as a raised elliptical area 310 disclosed in FIG. 3. The pressure applied by the upper embossing roller 460 and lower embossing roller 462 will allow any of the patterns disclosed herein to be introduced into the polymeric material.

Referring now to FIGS. 5A, 5B, 5C, 5D, and 5E, other embodiments that are contemplated by the present invention are disclosed. In particular FIG. 5A discloses an angled ellipses configuration 510 as opposed to the configuration disclosed in FIG. 3. The primary performance difference between FIG. 3 and FIG. 5A is that the angled ellipses configuration 510 has a tendency to force the propagating tear 70 in the direction of the angled ellipses, which in FIG. 5A is from left-to-right assuming the tear 70 is propagating in the general direction from top to bottom.

A similar alternating angled ellipses configuration 520 is shown in FIG. 5B. However, in this configuration, adjacent rows, for example rows 330 and 332, have elliptical areas that are oriented in opposite directions as shown. In this case, the propagating tear 70 has a tendency to travel a more tortuous path than either the basic configuration 300 of FIG. 3 or the angled ellipses configuration 510 of FIG. 5A

Referring now to FIG. 5C, an offset ellipses configuration 530 is depicted. In this offset ellipses configuration 530, the rows 330, 332, 334 are offset from one another. As with the angled ellipses configurations 520, 510, the offset between adjacent rows may be altered to achieve varying degrees of offset, such as rows 330 and 332, depending on the width of the neutral areas 320, 322 and the size of the raised and recessed elliptical areas 310, 312.

FIG. 5D shows yet another configuration, in particular a non-uniform configuration 540. In particular each row, 330, 332, 334, 336 is formed from a plurality of alternating raised elliptical areas 310 and recessed circular areas 512. In this non-uniform configuration 540, it may even be possible to have the neutral areas 320, 322, 324 run in a slight zigzag pattern if the raised elliptical areas 310 of adjacent rows, for example 330 and 332, extend far enough into the neutral areas 320, 322, 324 that the neutral areas 320, 322, 324 become somewhat S-shaped.

Referring now to FIG. 5E, a plurality of irregular S-shaped raised areas 552 and a plurality of irregular S-shaped recessed areas 554 are depicted forming rows 330, 332, 334, 336 in an example S-shaped configuration 550. The propagation of a tear 70 can be even more tortuous in the depicted S-shaped configuration 550 than in some of the previous configurations. Although not shown, it may be desirable to reverse the configuration of alternating rows, for example reversing the orientation of rows 332 and 336, similar to the reversed orientation of FIG. 5B. As illustrated by the numerous potential configurations of elliptical areas 310, 312 and S-shaped areas 552, 554, a person having skill in the art could mix and match features of the varying depicted configurations without departing from the scope of the present invention. Accordingly, based on the embodiments of the present invention that have been illustrated in the accompanying figures and described within this written description, it will be understood that the invention is not limited to the embodiments disclosed herein, but is capable of other rearrangements, modifications, and substitutes of parts and elements without departing from the spirit of the invention. 

1. A plastic bag for holding trash, debris and/or other materials comprising: at least one polymeric panel defining the outer walls of the plastic bag, the at least one polymeric panel interconnected to form a substantially cylindrical shape, a first end of the substantially cylindrical shape sealed to form a bottom of the plastic bag and a second end forming an opening of the plastic bag, the at least one polymeric panel defining a machine direction and a cross direction perpendicular to the machine direction; a plurality of substantially parallel rows on the at least one polymeric panel, the plurality of substantially parallel rows comprised of a plurality of curvilinear areas wherein each of the plurality of curvilinear areas is a protrusion or an indentation in the at least one polymeric panel; wherein each of the plurality of substantially parallel rows extends in the cross direction of the at least one polymeric panel.
 2. The plastic bag of claim 1 further comprising a plurality of substantially parallel neutral areas, each substantially parallel neutral area defined by an area between two adjacent rows in the plurality of substantially parallel rows
 3. The plastic bag of claim 1 wherein the plurality of curvilinear areas are elliptical.
 4. The plastic bag of claim 1 wherein the plurality of curvilinear areas are S-shaped.
 5. The plastic bag of claim 1 wherein the plurality of curvilinear areas are a series of alternating protrusions and indentations.
 6. The plastic bag of claim 1 wherein the plurality of curvilinear areas are substantially uniform in size and depth.
 7. The plastic bag of claim 1 wherein the plurality of curvilinear areas and plurality of substantially parallel neutral areas are not visually distinct.
 8. A plastic sheet with increased resistance to tear propagation comprising: a plastic sheet having an upper surface, a lower surface, and a plurality of edges, wherein the plastic sheet has an ordinary elasticity and the plastic sheet defines a machine direction and a cross direction perpendicular to the machine direction; a plurality of substantially parallel rows defined on the plastic sheet, the plurality of substantially parallel rows traversing the plastic sheet generally in the cross direction, the plurality of substantially parallel rows comprised of a plurality of curvilinear areas wherein each of the plurality of curvilinear areas is a protrusion or an indentation in the plastic sheet
 9. The plastic sheet of claim 8 further comprising a plurality of substantially parallel neutral areas, each substantially parallel neutral area defined by an area between two adjacent rows in the plurality of substantially parallel rows.
 10. The plastic sheet of claim 8 wherein the plurality of curvilinear areas are elliptical.
 11. The plastic sheet of claim 8 wherein the plurality of curvilinear areas are S-shaped.
 12. The plastic sheet of claim 8 wherein the plurality of curvilinear areas and plurality of substantially parallel neutral areas are not visually distinct.
 13. A method of manufacturing plastic film having increased resistance to tear propagation comprising the steps of: providing a blown-film polymeric tube, a machine direction defined by the longitudinal axis of the blown-film polymeric tube and a cross direction defined by an axis perpendicular to the longitudinal axis of the blown-film polymeric tube; flattening the blown-film polymeric tube; embossing the blown-film polymeric tube with a plurality of curvilinear areas, each curvilinear area being either a protrusion or an indentation;
 14. The method of claim 13 wherein the plurality of curvilinear areas are arranged into a plurality of substantially parallel rows extending in the cross direction of the blown-film polymeric tube.
 15. The method of claim 14, wherein the plurality of substantially parallel rows further define a plurality of substantially parallel neutral areas, each substantially parallel neutral area defined by an area between two adjacent rows in the plurality of substantially parallel rows.
 16. The method of claim 13, further comprises the steps of forming a plurality of plastic bags from the blown-film polymeric tube.
 17. The method of claim 13, wherein embossing the blown-film polymeric tube comprises the step of passing the blown-film polymeric tube through a pair of complementary embossing rollers.
 18. The method of claim 13, further comprising the step of: redirecting a tear propagating through the blown-film polymeric tube when the tear encounters one of the plurality of curvilinear areas. 